Andras Zsom, Sara Seager, Julien de Wit
We explore the minimum distance from a host star for an exoplanet to be potentially habitable, in order to maximize future chances of finding other habitable worlds. We find that the inner edge of the Habitable Zone (HZ) for hot desert worlds is at 0.5 AU around a solar-like star (well within the orbit of Venus). The relative humidity is the key controlling factor in determining the inner edge distance because water vapor has a strong impact on the greenhouse warming of the atmosphere, yet too little water vapor will deactivate precipitation and enable CO2 to accumulate. We estimate that a relative humidity as low as 1% can be sufficient to maintain a liquid water cycle and wash out CO2 from the atmosphere. If the surface pressure is too low (~0.1 bar), the water loss timescale of the planet is too short to support life. If the surface pressure is too high (~100 bars), we show using atmospheric circulation arguments, that the day-night side temperature difference on slow rotators and tidally locked planets is too small to enable an active water cycle. In contrast, the temperature difference on fast rotators with high surface pressure can be large enough to produce rain. Intermediate surface pressures (~1-10 bars) can provide suitable conditions for a water cycle independent of the planetary rotation period. We additionally find that the water loss timescale is influenced by the atmospheric CO2 level, because it indirectly influences the stratospheric water mixing ratio. If the CO2 mixing ratio of dry planets is smaller than 10^{-4}, the water loss timescale is ~1 billion years, which may be too short for complex life to evolve. (abridged)
View original:
http://arxiv.org/abs/1304.3714
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